Throughout its history, the golf ball has undergone an extensive evolution in an effort to improve its play-related characteristics such as durability, distance, and control. The first golf balls appeared around the 14th century and consisted of a leather sack which had been soaked in water and subsequently filled with wet feathers. As the sack dried, the leather would contract and the feathers expanded, resulting in a very hard mass which was then hammered into a round ball. These "feathery" golf balls were in use until about the mid 19th century, when a single piece golf ball made from "gutta percha" was introduced. Gutta percha is a rubber like material which is made from a type of tree sap, and at the time was primarily used as a packing material. The gutta percha balls were found to be more durable than the feathery balls, but did not offer the same distance as the older balls.
After the introduction of the gutta percha balls, it was discovered that a ball's flight was notably longer and more accurate after the surface had been nicked and scratched during use. Accordingly, golf ball manufacturers developed a number of methods for providing balls with a gutta percha cover that had a variety of patterns of nicks or markings on the surface. This eventually lead to the complex dimple patterns which are in use on golf balls today.
In the early 1900's, the wound rubber ball was introduced, which was considered a major breakthrough in the design of golf balls. It consisted of a ball in which a rubber thread was tightly wound around a solid rubber core, which wound core was then encased in a gutta percha cover. The early wound rubber balls lacked uniformity in size and weight, but the ball had a livelier core and the golfer was better able to control the spin of the ball.
Modern day golf balls can be classified as one-piece, two-piece, and wound golf balls. One-piece balls are molded from a homogeneous mass of material with a dimple pattern molded therein. Such balls are inexpensive and very durable, but do not provide great distance because of their relatively high spin rate and low velocity. Two-piece balls are made by molding a cover about a solid core. These are the most popular types of balls in use today. These balls typically have a hard cut-resistent cover which gives a high distance ball, but one which has lower spin rates resulting in a decreased ability to control the ball. Wound balls are made by molding a cover about a wound core. The core is typically made of rubber and can be solid, semi-solid or have a liquid center. A wound core is prepared by winding a lengthy thin thread of elastic material about the center core. The wound core is then covered with a durable cover material. Wound balls are generally softer than one-piece and two-piece balls and provide more spin, resulting in increased control over the ball, but typically travel a shorter distance than a two piece ball. As a result of their more complex construction, wound balls generally require a longer time to manufacture and are more expensive to produce than two-piece balls.
The covers of "modern" golf balls are made from a variety of materials such as Balata, SURLYN, IOTEK and polyurethane. Balata (trans-polyisoprene) is a type of natural rubber and is one of the softest cover materials. For many years, balata was the standard cover stock material for most golf balls. Balata covered balls are favored among professionals and more advanced amateur players because the softness of the cover allows the player to achieve spin rates sufficient to more precisely control ball direction and distance, particularly on shorter approach shots.
However, balata covered balls are expensive and less durable as compared to the other covering materials. In particular, balata covered balls are susceptible to nicks or cuts as a result of a poorly swung golf club, which is not uncommon with the average recreational golfer. Such nicks or cuts detract from the flight characteristics imparted by the dimple patterns on the ball, rendering them of little use. Accordingly, alternate cover compositions have been developed in an attempt to provide spin rates and a feel approaching those of balata covered balls while also providing a golf ball with a higher durability and overall distance.
In the middle 1960's, E.I. DuPont de Nemours and Co. discovered a new species of resins known as ionomer resins. These resins are sold under various trademarks such as SURLYN.TM. and, to a large extent, have replaced balata as a cover stock material. Chemically, these ionomer resins are a copolymer of an olefin and an alpha, beta ethylenically unsaturated carboxylic acid with 10-90% of the carboxylic acid groups being neutralized by a metal ion. See U.S. Pat. No. 3,264,272. Presently, the only commercially available ionomer resins are copolymers of ethylene and methacrylic or acrylic acid. These ionomer resins are distinguished by the type of metal ion, the amount of acid, and the degree of neutralization.
Dunlop Rubber Company obtained the first patent on the use of SURLYN.RTM. for the cover of a golf ball, see U.S. Pat. No. 3,454,280 issued Jul. 8, 1969. Since then, there have been a number of disclosures on the use of these ionomer resins in the cover composition of a golf ball. See, for example, U.S. Pat. Nos. 3,819,768 issued Jun. 25, 1974; 4,323,247 issued Apr. 6, 1982; 4,526,375 issued Jul. 2, 1985; 4,884,814 issued Dec. 3, 1989; and 4,911,451 issued Mar. 27, 1990. However, while these golf balls possess virtually cutproof covers, they have inferior spin and feel properties as compared to balata covered balls.
In November 1986 DuPont introduced a sodium and zinc ionomer resin having a low flexural modulus and suggested using and blending the same with other ionomer resins for making a golf ball cover. Golf ball covers made from these low flexural modulus ionomer resins have improved spin and feel characteristics, but relatively low velocity.
In December of 1986, DuPont introduced a lithium ionomer resin which was a copolymer of ethylene and methacrylic acid. These lithium ionomer resins have a very high flexural modulus, typically about 60,000 psi (415 MPa). DuPont suggested that lithium ionomer resins could be used to produce a golf ball cover which would be more cut resistant and harder than a cover made with either sodium or zinc ionomer resins. DuPont also suggested that a golf ball having a cover made from a lithium ionomer resin would go farther, have a higher coefficient of restitution and be less prone to cutting (i.e., more durable) than a golf ball made from other known ionomer resins such as sodium and zinc ionomer resins and blends thereof. DuPont further suggested that lithium ionomer resins could be used in blends with other ionomer resins where they can impart better cut resistance to those other resins.
The United States Golf Association (USGA) has promulgated a rule that no golf ball shall have an initial velocity that exceeds 255 feet (78 m) per second, i.e., 250 feet (76 m) per second with a 2% tolerance. Golf balls with covers made from ionomer resins with low flexural modulus are substantially below this maximum and, as should be appreciated, all golf ball manufacturers strive to come as close as possible to this limit.
In order to approximate the characteristics of balata covered balls at a lower cost, especially their softness and "feel", the art has developed balls of a variety having cover compositions.
It is known to add a softening agent such as n- or iso-butyl-acrylate to a relatively hard ionomer in order to obtain a softer ionomer. Cover compositions can be formed from blends of such soft and hard ionomers. As noted above, these prior art compositions have a considerably higher cut resistance and durability as compared to balata covered balls. However, despite numerous attempts to replicate the performance of balata covered balls, the golf ball cover compositions of the prior art generally suffer from low spin rates which makes them difficult to control near the greens. Further, such prior art balls tend to have relatively poor "click and feel" as compared to the balata covered balls. Additionally, many of the prior art golf ball cover compositions are low flexural modulus ionomer resins which have improved spin and feel characteristics, but relatively low velocity, which results in shorter overall distance.
Relatively recently, a number of golf ball manufacturers have introduced multilayer golf balls, i.e., balls with multiple core layers, multiple intermediate or mantle layers and/or multiple cover layers, in an effort to overcome some of the undesirable aspects of conventional two-piece balls such as their hard feel, while maintaining the positive attributes of these balls such as their increased initial velocity and distance. The physical characteristics of the cover layer and the various intermediate layers will vary depending upon the construction. Typically, one or more of the layers is softer in relation to the other layers. Examples of multilayer balls include the Episode (Titleist), Altus Newing (Bridgestone), Giga (Spalding), Metal Mix (Dunlop) and Ultra Tour Balata (Wilson).
Accordingly, there is a continuing need for new materials and additives for materials which impart desired properties, including softness, to golf ball covers and/or intermediate layers formed therefrom.